The invention relates to a technique for compensating for polarisation mode dispersion in optical transmission fibres. The invention also includes an optical transmission fibre incorporating means to compensate for polarisation mode dispersion. The invention further relates to an optical signal when subjected to such a fibre and to an equaliser for effecting such compensation.
Optical transmission fibres for use in optical transmission systems typically comprise a protective jacket surrounding a lower index cladding material enclosing a higher index optical transmission medium capable of supporting optical transmission along its length with little or no signal loss. Light is confined in the core region within such conventional step index fibres by total internal reflection as a result of the step difference in refractive index between the core and the cladding. The fibre and its cladding are near perfect circles in cross-section.
However, manufacturing tolerances, conditions of use, environmental factors and so on, give rise to imperfections in the geometric circularity of the fibres. This creates birefringence in the fibre, resulting in orthogonal polarisation states travelling at different group velocities within the fibre. Over long distances, the delay can be sufficiently cumulative that the original optical signal is effectively scrambled. This type of distortion, where orthogonal polarisation states travel at different group velocities, is known as polarisation mode dispersion (PMD).
Previous attempts at overcoming or compensating for PMD have employed either or both electrical and optical compensation techniques. Electrically based approaches are constrained to operate after the optical signal has been processed through an optical receiver (post-receiver) and are dependent on the speed of the electronics. Such electrical approaches have been found to be unsuitable for long-haul fibre links.
Optical PMD compensation techniques tend to focus on splitting the orthogonal polarisation states and delaying one relative to the other using free space optics or other optical components or, alternatively, writing a non-linear grating onto a high birefringence optical fibre. This latter approach has met with some success. The present invention aims to improve PMD compensation.
According to a first aspect of the invention, a method for compensating for polarisation mode dispersion in a birefringent optical transmission fibre, comprises controlling the birefringence of the fibre.
In a second aspect, the invention comprises a birefringent optical transmission fibre and means for controlling the birefringence of said fibre, whereby to compensate for polarisation mode dispersion in said fibre.
Polarisation mode dispersion control is preferably effected by writing a non-linear fibre grating in the fibre whereby to provide a means for imposing a differential time delay to the orthogonal polarisation states arising from the effects of polarisation mode dispersion such as to compensate for said polarisation mode dispersion.
The birefringence may be imposed in the fibre by a selection of any of the following options, namely introducing correctly positioned holes in the fibre so as to create a side hole fibre (SHF), a holey fibre (HF), a photonic crystal fibre (PCF) or any other suitable microstructure fibre.
The fibre grating is preferably a chirp or apodisation type grating or may be of any other suitable design. Variations in the properties of the birefringence and/or the grating are preferably performed by mechanical, electrical, thermal or acoustic means or methods.
The fibre can be tapered over part or all of its length. The micro-holes in the fibre may be filled with thermally sensitive material to create stressing rods in order to impose a mechanical stress by which to control birefringence. A thermal gradient can be provided over all or part of the length of the fibre. The fibre can be constructed in such a way that the fundamental transmission mode interacts with any in-fill material in the micro-holes so as to induce electro- or thermo-optic effects or voltage induced refractive index changes, or similar refractive index altering effect, in either or both of the fibre or the in-fill material that can be utilised to alter the mode shape and, consequently, the birefringence of the fibre. Any combination of these forms of fibre construction and birefringence control can be used in accordance with the invention.
Control of birefringence can be enhanced by providing additional means, such as stress rods, to alter the stress within the fibre and thereby the modal field pattern and therefore the birefringence of the fibre.